The use of jet propulsion devices for marine craft is well known technology. Jet propulsion has many advantages over the simple propeller, particularly in terms of maneuverability, and jet propulsion energy consumption is much more efficient. However, widespread acceptance of jet propulsion for marine craft has not occurred because of certain common problems associated with marine jet propulsion. For example, marine jet propulsion poses significant design problems because of uncertain performance over a wide range of speeds, water depth, sea conditions, etc.
Excess water pickup at the jet propulsion unit inlet may cause balling, i.e., excess water pressure between the hull and the inlet because the unit is not able to intake a sufficient volume of water during craft maneuvers or poor sea conditions. Balling induces a high drag characteristic adversely affecting the propulsive efficiency.
Cavitation is another common problem. Cavitation represents an uneven load on the impeller. Cavitation can be produced by excessive radial acceleration of the fluid, excess swirl and turbulence of the fluid column, and unintentional partial vaporization of the fluid throughput associated with a vacuum produced by impeller action.
Accordingly, it would be desirable to design a jet propulsion unit for marine vessels where each feature synergistically works together to provide for a constant column of water even at high output and where the water throughput is neither turbulent nor swirling in order to eliminate cavitation effects. Furthermore, the unit should have maximum flexibility to cope with the entire speed range of the marine vessel and varying loading on the unit without producing the above-mentioned balling and cavitation effects.
Finally, the unit ought to be efficient at preventing intake of foreign matter, yet have provided therefor a quick means for manually cleaning the intake if fouling occurs.
U.S. Pat. No. 4,449,944 to Baker et al. discloses a variable inlet device for a hydrojet boat drive permitting efficient transition from low to high speed operation of the boat. Installed in the "slot" of a "V" bottomed hull, the drive features a low drag ram-scoop with a blow-in door or panel which is responsive to imbalance between internal flow pressure and external slipstream pressure.
U.S. Pat. No. 3,543,713 to Slade discloses a propulsion unit for a marine vessel which operates by discharging water from a pump through an orifice. The orifice can be directed in accordance with the desired direction of propulsion.
U.S. Pat. No. 3,680,315 to Aschauer et al. discloses a hydraulic jet propulsion apparatus for boats having a variable area discharge nozzle.
Australian Patent Application 24907/88, filed Nov. 1, 1988 and opened to public inspection May 11, 1989, discloses a marine propulsion unit comprising a housing with a variable inlet induction, first set of vanes downstream of said induction, a propeller/impeller, a second set of vanes downstream of said propeller and a convergent discharge housing downstream of said second set of vanes. The use of a variable inlet orifice induction is said to reduce choking within the induction, and therefore cavitation and drag. The marine propulsion unit may be used with either outboard or sterndrive power trains.
U.S. Pat. No. 3,302,605 to Kuether discloses a jet propulsion apparatus for water craft which possesses a steering mechanism said to provide increased maneuverability and a structure of propeller and housing said to operate efficiently and requiring a minimum amount of power.
U.S. Pat. No. 3,187,708 to Fox discloses a jet propulsion unit for boats entirely outside of the hull that supplants the gear box propeller and rudder structure of the usual power boat arrangements.
U.S. Pat. No. 3,993,015 to Klepacz et al. discloses a hydraulic propulsion system for watercraft involving the forming of a parallel-sided, open-ended intake tunnel.
Other U.S. Pat. Nos. of interest include 3,889,623 to Arnold; 3,827,390 to De Vault et al.; 3,233,573 to Hamilton; 4,133,284 to Holcroft; 3,868,833 to Noe et al.; 4,652,244 to Drury; 3,192,715 to Engel et al.; 3,598,080 to Shields; 3,620,019 to Munte; 3,842,787 to Giacosa; 3,624,737 to Keller; 4,718,870 to Watts; 4,643,685 to Nishida; 4,600,394 to Dritz; 3,782,320 to Groves Jr.; 3,776,173 to Horwitz; 3,589,325 to Tattersall; 4,432,736 to Parramore; 3,788,265 to Moore; 4,474,561 to Haglung; and 4,925,408 to Webb et al.